In cellular radio communications systems, it is important to avoid simultaneous transmission from User Equipments, UEs, also known as Mobile Stations, MS, and Radio Base Stations, RBSs, especially if the transmission are on or near the same frequency and are in the same geographical area.
One reason for this is that the RBSs transmit with a much higher output power than the UEs. Additionally, the propagation conditions between the RBSs are typically different as compared to the propagation conditions between the RBSs and the UEs. For example, there may be line-of-sight between two RBSs antennas, whereas there is typically not line-of-sight between more than one RBS and a UE.
Thus, an RBS in one cell, which transmits on the same frequency as a UE in a neighbouring cell, may therefore interfere with the typically much weaker UE signal, so that the RBS in the neighbouring cell, for which the transmissions from the UE are intended, cannot decode the information in the radio communication from the UE. This phenomenon is known as “RBS to RBS interference”.
In a system which utilizes the so called TDD principle, (Time Division Duplex), during specified time periods, the up link frames, only transmission from the UEs to the RBSs is allowed, and during other specified time periods, the downlink frames, only transmission to the UEs from the RBSs is allowed.
One well-know method of avoiding “RBS to RBS interference” in TDD systems is to separate all up link transmissions in the system or at least in a group of cells in the system, from the down link transmissions in the system or in the group of cells. By alternating the system, or the group of cells, between uplink and downlink frames, the UEs and RBSs can communicate while avoiding the RBS to RBS interference problem.
Additionally, up link and down link frames are separated from each other by guard periods to account for e.g. propagation delays, timing advance and hardware switching times between receive and transmit. In current TDD systems, such as UTRA TDD and WiMAX, the guard periods are typically given by the standard and the same guard period is used in all of the cells in the system. Also, the guard periods are generally chosen to match the maximum round trip propagation delays between a base station and terminals at the border of the cell of the base station.
An additional factor to take into account in TDD systems is the non-zero propagation time of the radio signals transmitted from the RBSs. Due to this phenomenon, transmissions from one RBS will arrive at other RBSs slightly delayed with respect to the transmission time in the first RBS. Thus, the guard periods must to some extent also be chosen so that down link transmissions from one RBS do not coincide with the up link periods in other RBSs.